MRI uses a strong magnetic field and radio waves instead of X-rays to produce images showing structural and chemical changes in tissues. The patient lies on a table in a narrow cylinder (the scanner) containing a magnetic field and images are taken. A contrast agent called gadolinium is injected into a vein during the last set of images to help identify new lesions. Magnetic resonance spectroscopy, which is similar to MRI, is also done once during the baseline phase, at 4 months and at 8 months to measure brain chemicals. For the spinal tap, a local anesthetic is given and a needle is inserted in the space between the bones (vertebrae) in the lower back. About 2 tablespoons of fluid is collected through the needle. For leukapheresis, whole blood is collected through a needle placed in an arm vein. The blood circulates through a machine that separates it into its components. The white cells are removed and the red cells, platelets and plasma are returned to the body through a second needle placed in the other arm.